This invention relates to antenna system. In particular, this invention relates to antenna systems used for simulcast communication systems.
Simulcast communication systems expand communications coverage in a geographic region by using multiple antennas with radio transmitters at each antenna to simultaneously or nearly simultaneously broadcast a program signal from each antenna that originates from a program source distant from the antennas. A problem with simultaneously broadcasting a program signal from separate radio transmitters at separate antennas occurs when a receiver receives signals from more than one antenna. Signals at a receiver from different antennas that are out-of-phase at the receiver makes demodulation of the signals difficult.
When voice or analog signals are simulcast from multiple antennas, simulcast system designers must minimize modulation differences between transmitter sites and must minimize phase differences throughout the geographic region of coverage by positioning transmitting stations and antennas appropriately. In many cases, time delays must be added to a signal for broadcast to insure that signals from different antennas do not destructively combine at some point within the geographic region.
When digital signals, such as binary data or digitized voice, are simulcast from multiple antennas, phase differences between signals received from multiple antennas present a critical problem. Since digital signals are susceptible to additive and destructive wave interference, the requirements of phasing and amplitude equalization of signals broadcast from multiple antennas becomes even more critical. Digital signals from multiple transmitters that are out of phase at a receiver may render data unrecoverable by a receiver. Bit errors that occur in demodulating a digital signal that are caused by a receiver unable to discriminate against other signals increase a phase differences between signals from two or more simulcast antenna sites increase and magnitude differences between the RF signals decrease.
Prior art digital simulcast systems typically increase the number of transmitting sites in a geographic region to insure that virtually no area in a geographic region of coverage is without an RF signal from at least one transmitting site that is sufficiently large in amplitude with respect to other signals to overcome phase differences between RF signals received from other transmitting sites located elsewhere in the region. Increasing the number of transmitter sites, however, to insure accurate demodulation and accommodate increasing data rates becomes prohibitively expensive, complex, and decreases overall system reliability. An antenna system specifically for use with digital data simulcast networks that reduces the need for multiple antenna sites while increasing the reliability of the communication system would be an improvement over the prior art.